Journal of Non-Crystalline Solids 34 (1979) 81-99 North-Holland Publishing Company

THE STRUCTURE OF LITHIUM, SODIUM AND POTASSIUM GERMANATE GLASSES, STUDIED BY RAMAN SCATTERING

H. VERWEI J

Pbilips Research Laboratories, Eindhoven, The Netherlands

and

J.H.J.M. BUSTER

Philips Semiconductor Devices Factory, Nijmegen, The Netherlands

Received 7 February 1979 Revised manuscript received 12 April 1979

Polarized and depolarized Raman spectra of alkali germanate glasses are given, together with Raman powder spectra of the crystalline compounds Li20 - 2 GeO2 ; 3 Li20 8 GeO2 ; 2 Li20 - 9 GeO2; Li20 7 GeO2; 2 Na20 9 GeO2;K20 2 GeO2;K20 - 4 GeO2 and K20 8 GeO2.

The alkali germanate glasses: xA20 (1 - x) GeO2 are studied in the composition range 0 < x < 0.333. The vibrational modes observed in the high energy range of the Rarnan spectra of the crystalline compounds are interpreted in terms of symmetrical and antisymmetrical O Ge-O and Ge -O- stretch vibrations. The molecular structure of the germanate glasses is deduced from a comparison of the Raman spectra of the glasses with those of the crystalline compounds, together with a study of the polarization properties of the glass spectra.

It is observed that 6-coordinated Ge atoms occur in a network structure which resembles the structures occurring in the crystalline compounds 2 Li20 - 9 GeO2 and 2 Na20 9 GeO2.

In the region of 0.18

82 11, Verweif, J.H.J.M. Buster/Raman spectra of alkali germanate glasses

tion show relatively large differences in refractive index compared to alkali silicate glasses in the composition region around 20 mol % alkali oxide. This makes alkali germanosilicate glasses particularly suitable for the production of compound glass fibres for optical communication with an index gradient based on alkali ion diffu- sion between core (sodium-containing glass) and cladding (potassium-containing glass) [3-5].

The differences in the refractive indices have been shown to be caused by the germanate structures in the germanosilicate glasses [5].

In contrast to alkali silicate glasses, where the refractive index is a monotonously increasing function of the alkali content, alkali germanate glasses exhibit a maxi- mum in refractive index as a function of the alkali content [6,7]. The maximum has tentatively been explained as being due to the occurrence of a change of coordination of Ge by O [6,7]. No detailed proposals about the germanate network structure have been reported yet.

The present paper proposes a possible network structure for alkali germanate glasses based on Raman spectroscopic observations.

1.1. Structural studies on vitreous Ge02

Structural studies on vitreous GeO2 are numerous and a review can be found in [8]. Besides the vitreous modification of GeO2, a trigonal phase with a quartz-like structure [9,10]. a tetragonal phase with the rutile structure [11-13] and a pseu- do-cubic phase having the cristoballite structure [14] are known. In the trigonal and pseudo-cubic phases the coordination number of Ge is 4 and in the tetragonal phase the coordination number of Ge is 6.

From X-ray diffraction [15,16], neutron diffraction [17,18], combined X-ray neutron diffraction [19] and EXAFS [20] studies it is concluded that the coordi- nation of Ge in vitreous GeO2 is fourfold and corresponds more or less with the coordination in the trigonal modification. This suggests a three dimensional random network structure consisting of connected GeO4 tetrahedra for vitreous GeO2.

The structural correspondence between trigonal GeO2 and vitreous GeO2 has also been concluded from IR transmission spectra [21-24]. In vitreous GeQ a

Table 1 Observed Raman frequencies of tetragonal (rutile type) GeO 2 [28] together with a description of the atomic movements of the GeO6 octahedra [29].

Freq. (cm -1 ) Symmetry Description

170 B 1 g Torsion 680 Eg Antisymmetric bending/stretch 702 Alg Symmetric bending/stretch 870 B2g Symmetric bending/stretch

H. Verwei], J.H.J.M. Buster/Raman spectra of alkali germanate glasses 83

Table 2 Observed Raman frequencies of trigonal (quartz like) GeO2 [28] and vitreous GeO2 [27] together with a description of the atomic movements of the GeO 4 tetrahedron [ 31 ].

GeO2 trigonal Symmetry GeO 2 vitreous Assignment freq. (cm -1 )

Freq. (cm -1) Intensity a) dep. ratio

121 E(TO + LO) 60 b, s, d 166 E(TO + LO) 212 A 1 26 l A1 326 E(TO) 320 m, p 372 E(LO) 385 E(TO) 440 A I 412 s, p 456 E(LO) 560 b, m, p 492 E(TO) 512 E(LO) 583 E(TO) 595 E(LO) 857 E(TO) 860 w, d 880 A I 949 E(LO) 945 w, d 961 E(TO) 972 E(LO)

Bond rocking Bond rocking

Bond stretching

Bond stretching

a) b = broad, s = strong, m = medium, w = weak, p = polarized, d = depolarized.

broad Ge-O-Ge stretching band is found at 895 cm -t corresponding to the band at 885 cm -1 in trigonal GeO2. In tetragonal GeO2 the highest frequency band is found at 720 cm -~ [21]. Raman powder spectra of vitreous, trigonal and tetragonal GeO2 have been reported in [23]. Polarized and depolarized Raman spectra of vitreous GeO2 have been given in [25-27] . A Raman single-crystal study of both trigonal and tetragonal GeO2 in which all symmetry species could be assigned is reported in [28]. Like the IR spectra the Raman spectra suggest a structural similar- ity between vitreous and trigonal GeO2.

In table 1 the observed Raman frequencies of tetragonal GeO2 [28] are given together with a description of the corresponding movement of the oxygen atoms in the GeO6 octahedra as obtained from the normal coordinates reported in [29] for the rutile structure.

Bell et al. calculated density of states spectra for computer-constructed models of vitreous SiO2, GeO2 and BeF2 [30-32] . They gave quantitative estimates for the degree of localization [31] and the ratio of optical and acoustical character of the calculated spectra [32].

In table 2 the Raman frequencies of trigonal GeO2 and vitreous GeO2 observed in [27,28] are listed, together with the assignments given by Bell et al. [31].

84 H. Verweij, J.H.J.M. Buster / Raman spectra of alkali germanate glasses

1.2. Structural studies of alkali germanate compounds

1.2.1. Lithium germanates. Phase diagrams of the Li20-GeO2 system have been reported in [33] and [34]. The reported stable compounds are: Li20" 7 GeO2, 3 LifO 8 GeO2, [34], Li20 GeO2 [33,34], 3 Li20 2 GeO2 [33,34] and 2 Li20 GeOz [33,34]. The crystal structure of all these compounds, except for 3LizO 8 GeO2 and 3 Li20 2 GeOz, has been resolved by means of X-ray diffraction.

The structure of LizO 7 GeOz [35] contains strongly puckered layers of GeO4 tetrahedra linked by GeO6 octahedra forming a three-dimensional network. The structure can be characterized by the formula Li2 [Ge(G%Os)3]. The oxygen atoms occurring in the network are either bridging atoms between two tetrahedrally coor- dinated Ge atoms or bridging atoms between one tetrahedrally and one octahe- drally coordinated Ge atom.

The structure of Li20 " GeO2 [36,37] is isotypic with the structure of Li20 SiO2 [38] and consists of infinite chains of GeO4 tetrahedra.

The compound 2 Li20 GeO2 [39] contains GeO4 tetrahedra, linked by LiO4 tetrahedra.

The compound 2 Li20 - 9 GeO2 can be obtained by fast cooling of a melt of the same composition [40]. The crystal structure [40] of this compound contains chains of GeO4 tetrahedra, connected by GeO6 octahedra forming a three-dimen- sional network similar to the network in Li20" 7 GeO2 [35]. The structure of 2 Li20 9 GeO2 can be characterized by the formula Li4 [Ge2(Ge709)].

A compound of composition Li20 4 GeO2 has also been reported [41]. The structure of this compound is said to be similar to the structure of the compounds Li20 - 7 GeO2 [35] and 2 Li20 - 9 GeO2 [40].

According to [41] the compound 3 Li20 - 8 GeO2 reported in [34] consists of a mixture of Li20 GeO2 and Li20 - 4 GeO2.

The compound Li20 2 GeOz, which can be prepared from a glass with the com- position Li20 "2,25 GeO2, nucleated with crystalline Li20 2 SiO2 has also been the subject of a crystal structure study [42,43]. The structure is isotypic with Li20 - 2 SiO2 [44] containing puckered layers ofGeO4 tetrahedra.

The compound 3 Li20 2 GeO2 [45] was reported to be structurally similar to 3 Li20 2 SiO2 [46] and to contain groups of two GeO4 tetrahedra sharing one corner.

1.2.2. Sod&m germanates. Phase studies of the sodium germanate system have been reported in [47] relating to the compounds Na20 4 GeO2 and Na20 - Ge02, in [48] relating to the compounds Na20 - 4 GeO2, NazO 2 GeO2 and Na20 GeO2 and in [49] relating to the compounds 2 Na20 9 GeO2 and NazO GeO2.

An X-ray single-crystal structure study of 2 Na20 9 geO2 has been reported in [50]. The structure contains chains of Ge04 tetrahedra, connected by GeO6 octahedra. The structure is not isomorphous with the structure of 2 Li20" 9 GeO2 [40] in which the oxygen atoms are all of the bridging type as in 2 Na20 - 9 GeO2.

H. Verwei], J.H.J.M. Buster / Raman spectra of alkali germanate glasses 85

A study of the crystal structure of Na20" GeO2 has been reported [51]. As in Li20. GeO2 [36,37] the structure consists of pyroxene-like chains of GeO4 tetra- hedra sharing two corners. A compound Na20" 4 GeO2 has also been reported [45,52-55] which is thought to be isotypic with L i20-4 GeO2 [41], LiNaO" 4 GeO2 [56] and K20 4 GeO2 [57].

From the observations described in [50-53] it can be concluded that the phase diagram of [49] is probably the most correct one.

1.2.3. Potassium germanates. Phase studies of the potassium germanate system have been reported in [48], mentioning K20 4 GeO2, K20 2 GeO2 and K20 GeO2 and in [58] mentioning K20"7GeO2, 3K20.11GeO2, K20"2GeO2 and 10 K20 11 GeO2 as stable compounds. The compositions reported in the latter study are probably incorrect and should be: K20 8 GeO2, K20 4 GeO2, K20 " 2GeO2 and K20. GeO2 [45], [521, [53], [57], [59] and [60].

An X-ray single-crystal structure study of K20 8 GeO2 has been reported in [60]. The structure contains a three-dimensional network which is somewhat sim- ilar to the feldspar structure. Three-quarters of the Ge atoms are surrounded tetra- hedrally by oxygen atoms, the other ones are present in 5-coordination in a dis- torted trigonal bipyramid. All oxygen atoms are of the bridging type. The structure of K20 4 GeO2, also studied by X-ray diffraction [57], is similar to that of Li20 4 GeO2 [41] and LiNaO 4 GeO2 [56]. The structure contains Ge309 rings con- sisting of three GeO4 tetrahedra; these rings are connected by GeO6 octahedra forming a three-dimensional network [57]. The oxygen atoms are all of the bridging type; the structure can be characterized by the formula K2 [Ge(Ge3Og)].

No X-ray structure studies of the compounds K20 ' 2 GeO2 and K20-GeO2 have been reported in the literature. In [61] it is concluded from the Raman spec- trum that the structure of K20" 2 GeO2, is similar to that of ki20" 2 GeO2 [42,43]. The same goes for K20"GeO2 [62] which is similar to Li20-GeO2 [36,37] and Na20 GeO2 [51].

Furthermore metastable potassium germanates have been reported with compo- sitions: K20 6 GeO2 [45,59] and 2 K20 9 GeO2 [45], [52] and [53], which is reported to be isomorphous with 2 Na20 9 GeO2 [50].

1.3. Structural studies on alkali germanate glasses

As pointed out, alkali germanate glasses exhibit a maximum in refractive index and density as a function of alkali content [7,63] which has been tentatively explained in terms of a coordination change of Ge. A structural interpretation of refractive index and density has been given in [5,64], where it is respectively con- cluded that 6-coordinated Ge atoms occur in a structure like that of 2 Na20" 9 GeO2 [50] or in a structure like that of K20 4 GeO2 [64]. The same conclu- sions were drawn from a thermoluminescence study of X-ray irradiated glasses [65]. IR studies of alkali germanate glasses are given in [24,55], where it is

86 H. Verweij, J.H.J.M. Buster / Raman spectra of alkali germanate glasses

reported that the main absorption band, due to a Ge-O-Ge stretch vibration, shifts to longer wavelengths with increasing amounts of alkali to a much greater extent than in silicates. Above 25-30 mol % alkali oxide this main absorption band splits into two.

From a comparison of the IR spectra of the glasses with those of vitreous, tri- gonal and tetragonal GeO2 it is concluded that the predominant mechanism for the shift of the main absorption band at alkali concentrations less than 25 mol % alkali oxide is a change in the coordination number of Ge 4+ from 4 to 6. The splitting of the band above 25 tool % alkali is ascribed to the formation of non-bridging oxygen atoms.

In [55] a tentative attempt is'made to interpret the shift of the main absorption band quantitatively in terms of concentrations of six-coordinated Ge. It was found that the maximum in concentration of Ge vl occurred at about 20 tool % for sodi- um and potassium germanate glasses and at about 15 tool % for lithium germanate glasses. The Ge vl concentrations found for the glasses lie in the same region as in the compounds Li20 7 GeO2, Na20 4 GeO2 and K20 4 C eO2 but are signifi- cantly lower than in the compound 2 Na20 9 GeO2.

Unpolarized Raman spectra of alkali germandate glasses have been given in [25,66]. Polarized and depolarized Raman spectra of alkali digermanate and alkali metagermanate glasses have been given respectively in [61] and [62]. In the Raman study of [66] it was concluded that alkali germanate glasses are structurally differ- ent from alkali silicate glasses, especially at low alkali concentrations. It was also concluded in [66] that glasses with the digermanate composition contain diger- manate structures as in K20 ' 2 GeO2 [42,43]. This same conclusion was also arrived at [61]. In [62] it was concluded from Raman spectra that alkali metager- manate glasses contain infinite chains of GeO4 tetrahedra as in the crystalline meta- germanates Li20 GeO2 [36,37] and Na20 GeO2 [51].

2. Experimental

The glasses were prepared from: Li2CO 3 * (reagent grade). Na2CO3 * (reagent grade, dried for 20 h at 300C)., K2CO3 * (reagent grade, dried for 20 h at 300C). SiO2 ** (a-quartz, milled rock crystal, sieve fraction 60-250/a). GeO2 *** (extra pure).

The molar compositions were: xA20 (1 -x ) GeO2 with x = 0.111, 0.150, 0.182,

* E. Merck Darmstadt, W. Germany. ** Hereaus, tlanau, W. Germany.

*** lloboken, Belgium.

H. VerweiL J.H.J.M. Buster / Raman spectra of alkali germanate glasses 87

0.200, 0.250 and 0.333. The compositions with x = 0.111, 0.182, 0.200 and 0.333 correspond to A20. 8GeO2, 2A20.9GeO2, A20 .4GeO2 and A20"2GeO2 respectively, where A = Li, Na or K.

The components were mixed followed by melting at 1200C in an electrically heated fumace. Oxygen was bubbled through for half an hour in order to homo- genize the melt. After bubbling the melt was allowed to become bubble-free. All lithium containing glasses and the glasses containing sodium or potassium with x = 0.111 and 0.200 were melted in Pt/10 Rh crucibles. Because of their reactivity towards Pt/10 Rh the sodium- and potassium-containing glasses with x = 0.333 were melted in A1203 crucibles *. Samples in the vitreous state were obtained by air quenching of 10 g portions or by pressing between two cold copper plates.

Details of the laser Raman set-up are given in [61,62], where a description of the measurement procedure for the very hygroscopic glasses with A = K and x = 0.333 is also to be found. The other samples were cut, ground and polished so that they had two parallel sides and one perpendicular flat side. The direction of the incident laser beam was perpendicular to the two parallel sides. The samples were positioned such that the laser beam passed very near the perpendicular side to avoid internal depolarization effects. The crystalline compounds Li20" 7 GeO2 [35],3 Li20" 8 GeO2 [34], K20 '8 GeO2 [60], K20" GeO2 [57], K20 "2 GeO2 [48] and 2 Na20.9 GeO2 [50] were prepared by sintering powdered glass or crystallized melt at temperatures just below the melting points of the compounds.

The compound 2 Li20 9 GeO2 was obtained by allowing a 1 g portion of the stoichiometric melt to crystallize during air cooling. Immediately after crystalliza- tion the sample was water-quenched to avoid further decomposition of the com- pound formed. Li20 - 2 GeO2 was prepared according to [42,43] from a glass pow- der with composition Li20 2.25 GeO2, nucleated with 5 wt. % crystalline Li20 - 2 SiO2.

The structure of the samples was checked by recording X-ray diffraction pat- terns which were compared, when available, with calculated X-ray diffractograms. These calculated diffractograms were obtained with the aid of a computer program and single-crystal data from the literature. The Raman spectra of the crystalline powders, contained in quartz glass capillaries of 1 mm inner diameter, were ob- tained using a laser power of 1 W.

Results

3.1. Crystalline alkali germanates

Raman powder spectra of the crystalline compounds Li20 2 GeO2, 3 Li20. 8GeO2, 2L i20-9GeO2, L i20"7GeOz, 2Na20-9GeO2, K20-2GeOz, K20- 4 GeO2 and K20 8 GeO2 are given in figs. 1 and 2. Spectral data together with assignments are given in tables 3 and 4. From inspection of the Raman spectra it

* Degussa, Frankfurt, W. Germany.

88 H. Verwei], J.H.J.M. Buster / Raman spectra of alkali germanate glasses

i I ! ~l t, g ~

Li207Ge02 %~JL~ ~ ~ ~ ~-~ ~ ~ . . . . .

2 ,2o. eo2

,.//8~-~ ~ a~ 3Li20-SGeO2 ~ ~

' [iii,,)il Li202Ge02~ V~_~ ~ ~ '~ UUt r

I I I I [ I 1200 1000 800 600 400 200

Av(crrd)

"d u)

o m m e a o I ~ ~

K20'8GeO 2 .I.~, ~ g ~ /'v ~N-' L'i ~1 ,vf>,wt ~o

N ~ N I O%oS. I K204Ge02

g

' I I I I I [ I

1200 1000 800 600 400 200 Av(cnd}

Fig. l . Raman powder spectra of L i20 2 GeO 2 , 3 L i20 8 GeO2, 2 L i20 9 GeO2 and Li20

7 GeO2.

Fig. 2. Raman powder spectra of 2 Na20 ' 9 GeO2, K20 ' 2 GeO2, K20 4 GeO2 and K20 .

8 GeO2.

was concluded that the compound 3 Li20 8 GeO2 is a real compound as argued in [34] and is not a mixture of Li20 - GeO2 and Li20 4 GeO2 as was suggested in [41].

Spectral data of crystalline Li20 - 2 GeO2 and K20 2 Ge02 together with assignments have already been given in [61]. In crystalline L i20-2 GeO2 and K20 2 GeO2, uGe-O- is found at 900 and 853 cm -~ respectively; usO-Ge-O is found as a group of peaks around 490 cm -1 for Li20 2 GeO2 and as a strong peak at 533 cm -1 for K20 2 GeO2. Peaks at 772,820 and 860 cm -1 in the Raman spec- trum of Li20 2 GeO2 and at 621, 732 and 840 cm -1 in the spectrum of K20 2 GeO2 are ascribed to uasO-Ge-O vibrations. The peaks below 400 cm -1 are ascribed to deformation, A -O stretch and lattice modes.

The interpretation of the Raman powder spectra of the compounds containing less alkali is difficult. Only a rough approximative assignment can be made from an analogy with the Raman spectra of the various GeO2 modifications (section 1.1)

H. Verwei/, J.H.J.M. Buster / Raman spectra of alkali germanate glasses 89

e,I

,..a

t-,i

-1

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xl

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I

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0"1 O0 0"1

.~'~ ~x

-~.~

90 H. Verwei], J.H.J.M. Buster / Raman spectra of alkali germanate glasses

oo

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O

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<

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11. VerweiL J.H.J.M. Buster / Raman spectra of alkali germanate glasses 91

and the digermanate and metagermanate compounds [61,62]. In the assignments given in table 3 and 4 a distinction is made between: vGe-O- - indicative of the presence of non-bridging oxygen atoms [61]. vsGeO 6 - symmetrical stretching mode of the oxygens around 6-coordinated

Ge atoms, comparable with the A1 mode observed in tetragonal GeO2 [28].

VsO-Ge-O, deformation, A -O stretching and lattice modes. vasO-Ge-O.

From [61] and [62] it is known that vGe-O- in A20 2 GeO2 and vsGe-O- in A20 GeO2 appear as strong peaks in the Raman spectrum in the region 800-900 cm-1; VasO-Ge-O appears as weak peaks in the region of 700-900 cm-1; and usO-Ge-O, deformation A -O and lattice modes appear below 600 cm -1.

The structure of 3 Li20 8 GeO2 has not yet been resolved. In the region of 700-900 cm -1 no strong vGe-O- peak is observed in the Raman spectrum (fig. 1) so that the absence of non-bridging oxygen atoms in the structure may be con- cluded. The most pronounced feature in the spectrum is a strong peak at 656 cm -x, which is tentatively ascribed to a symmetric stretch vibration of GeO6 octahedra present in the structure. This assignment is based on the analogy with the Raman spectrum of tetragonal GeO2 (table 1) in which a strong stretching mode is found at 702 cm -1, The group of less intensive peaks between 750 and 900 cin -1 is ascribed to vasO-Ge-O, in which Ge may be 4-coordinated (compare digermanates [61] and metagermanates [62], or 6-coordinated (compare the antisymmetric stretching mode at 870 cm -1 in tetragonal GeO: (table 1)).

In the Raman spectrum of 2 Li20 9 GeO2 (fig. 1) two intense peaks are found at 595 and 566 cm -1. The structure of 2 Li20 9 GeO2 contains chains of GeO4 tetrahedra connected by GeO6 octahedra and no non-bridging oxygen atoms (NBO's) [sect. 1.2.1 ; 40].

The intense peaks at 595 and 566 cm -~ can be assigned to: VsO-Ge-O of the chains, comparable to usO-Ge-O of metagermanate chains, which is found at 580-600 cm -1 or vsGeO6 of the GeO6 octahedron, comparable to the strong A~ mode at 702 cm -a in the Raman spectrum of tetragonal GeO2. The weak peaks in the Raman spectrum of 2 Li20 9 GeO2 between 700 and

950 cm -x can be assigned to UasO-Ge-O. In the Raman spectrum of Li20" 7 GeO~ (fig. 1) the strongest peak is found at 475 cm -1. The structure of LifO 7 GeO2 contains a highly condensed three-dimensional network of GeO4 tetra- hedra, connected by GeO6 octahedra [35] and bears some resemblance to the tri- gonal GeO2 structure [9,10]. The strong peak at 475 cm -x in the Raman spectrum is comparable to the strong peak at 440 cm -a which is found in the Raman spec- trum of trigonal GeO2 and which is due to an AI type deformation mode (table 2). The peaks at 802,858 and 925 cm -l in the Raman spectrum of Li20 7 GeO2 are ascribed to vasO-Ge-O.

The Raman spectrum of 2 Na20 " 9 GeO2 is given in fig. 2. The structure of

92 H. Verweij, J.H.J.M. Buster / Raman spectra of alkali germanate glasses

2 Na20" 9 GeO2 contains chains of GeO4 tetrahedra connected by GeO6 octa- hedra and no NBO's [50]. By analogy with 2 Li20 9 GeO: the strong peaks found in the spectrum at 648 and 608 cm -I are ascribed to UsO-Ge-O of the chains or t)sGeO6 of the octahedra. The peaks between 750 and 950 cm -1 are ascribed to UasO-Ge-O.

In the Raman spectrum of K20 " 4 GeO2 (fig. 2) the strongest peaks are found at 517 and 499 cm -1, The structure contains Ge309 rings of GeO4 tetrahedra con- nected by GeO6 octahedra and no NBO's [57].

The strong peaks at 517 and 499 cm -1 are probably caused by u s O-Ge-O ring modes.

The Raman spectrum of K20 8 GeO2 (fig. 2) resembles the spectrum of Li20 7 GeO2. The structure contains a three-dimensional network of GeO4 tetrahedra and GeOs groups and no NBO's [60]. As in the case of Li20 7 GeO2 the strong peak at 475 cm -1 is comparable to the 440 cm -1 peak found in the Raman spec- trum of trigonal GeO2.

To conclude this section we mention a number of general Raman spectral fea- tures of the germanates with a low alkali content.

A. Between 700 and 950 cm -1 weak peaks due to uasGe-O-Ge vibrations are found which can be thought to be derived from the F2 type uasGeO 4 vibra- tion of the GeO4 tetrahedron [61,62]. Most of these peaks are expected to be depolarized.

B. Enneagermanate structures (as found in 2 L i20 '9 GeO2 and 2Na20 ' 9 GeO2), containing chains of GeO4 tetrahedra connected by GeO6 octahedra, give rise to strong Raman peaks around 600 cm-1; they are ascribed to UsO-Ge-O of the chains or usGeO 6 of the octahedra. For both assignments it is expected that the peaks are strongly polarized. usO-Ge-O can be thought to be derived from the polarized UsO-Ge-O peak found in metagermanates [62] and usGeO 6 from the Al type mode found in the Raman spectrum of tetragonal GeO2 [28].

C. The spectral region below 600 cm -~ contains usO-Ge-O, which is expected to be polarized [61,62] and deformation, uA-O and lattice modes which may be partly polarized or depolarized.

3.2. Alkali germanate glasses

3.2.1. Lithium germanate glasses. Figure 3 gives the polarized and depolarized Raman spectra of lithium germanate glasses. The Raman spectrum of 0.333 Li20 0.667 GeO2 glass has previously been described in [61].

The broad intense bands observed in the polarized spectrum at 850 and 550 cm -1 are ascribed to uGe-O- and usO-Ge-O respectively. In the depolarized spec- trum the maximum of the highest energy band lies at 840 cm -l, this is related to the presence of UasO-Ge-O vibrations.

The spectrum of 0.250 Li20 0.750 GeO2 glass still shows the presence of a po-

H, Verweij, J.H.J.M. Buster / Rarnan spectra of alkali germanate glasses 93

(O) ~ Y(ZZ)X

I ] I I I I 1200 1000 800 6130 400 200 0

av(crrr-l)

(b) YIZYIX

i~ o J ' / ', / \ ~ /~

x=0.111 ~ '~/ ~ / /

x=0.150 / ~' \ / ~ / _

/ / x=0.182 ) ~ ~ j ~ '~

I I I 1 I I 1200 1000 800 600 /,00 200 0

Av(crrd)

Fig. 3. Polarized (a) and depolarized (b) Raman spectra of xLi20 (1 -x ) GeO2 glasses. Nota- tion for the scattering geometry according to [67]. Relative scale expansion of the depolarized Raman spectra: 10.

larized vGe-O- band; the other spectral features correspond to those found in the spectra of the glasses with x = 0.200, 0.182 and 0.150 and are summarized below.

A. In the region from 700 to 1000 cm -1 a contour of partly depolarized bands is observed (fig. 3b) which is ascribed to uasO-Ge-O.

B. A broad polarized band is found, situated at about 540 cm -1 (fig. 3a), which can be ascribed to UsO-Ge-O and to deformation modes, comparable to the deformation modes in vitreous GeO2, which give rise to a strong polarized band in the Raman spectrum of vitreous GeO2 (table 2).

C. Below 400 cm -1 a partly depolarized scattering continuum is found, probably caused by deformation, vA-O and lattice modes.

The Raman spectrum of 0.111 Li20 0.889 GeO2 glass has an appearance different from that of the other spectra; the observed bands are relatively sharp. The bands at 864 and 823 cm -1 are ascribed to uasO-Ge-O. The sharp polarized bands at 560 and 540 cm -1 are ascribed to usO-Ge-O or vsGeO ~ in enneagermanate structures (compare 2 Li20 "9 GeO2, section3.2). The bands at 435 and 410cm -1 are ascribed to deformation modes of the vitreous GeO2 type. Bands due to enneager-

94 H. VerweiL J.H.J.M. Buster / Raman spectra of alkali germanate glasses

manate structures are probably also present in the spectra of the other glasses. They are not visible because they form part of the broad band situated at about 540 cm -1"

3.2.2. Sodium germanate glasses. Figure 4 gives polarized and depolarized Raman spectra of sodium germanate glasses. The assignment of the spectra of 0.333 Na20 0.667 GeO2 glass was given in [61]. The polarized band at 870 cm -l is ascribed to uGe-O- of digermanate, the polarized band at 800 cm -1 to usGe-O- of metager- manate, the polarized band at 530 cm -1 to usO-Ge-O and the polarized bands at 600 and 653 cm -1 to vsO-Ge-O or vsGeO6 of the enneagermanate structure. The bands at 850 and 770 cm -1 found in the depolarized Raman spectrum of 0.333 Na20 0.667 GeO2 glass are ascribed to UasO-Ge-O.

The peak position and intensity of the peaks at 600 and 653 cm -l in the polar- ized Raman spectrum of 0.333 Na20 0.667 GeO2 glass agree remarkably well with the peak position and intensity of the peaks in the same region in the Raman spec- trum of crystalline 2 Na20 9 GeO2 at 608 and 648 cm -1 (see section 3.2).

The enneagermanate peaks are not observable in the other Raman spectra of fig. 4.

(a) /~Y(ZZ)X

L I I I I I 1200 1000 800 600 /,00 200 0 1200 1000 800 500 /.00 200 0

~v(crrd) Av(crn-l) Fig. 4. Polarized (a) and depolarized (b) Raman spectra of xNa2) (1 -x) GeO2 glasses. Nota- tion for the scattering geometry according to [ 67]. Relative scale expansion of the depolarized Raman spectra: 10.

H. Verwei], J.H.J.M. Buster / Rarnan spectra of alkali germanate glasses 95

A polarized uGe-O- band is found in the Raman spectra of 0.250 Na20 0.750 GeO2 and 0.200 Na20 0.800 GeO2 glass at 867 cm -1. Furthermore the following general spectral features are observed:

A. In the region from 700-1000 cm -1 a contour of partly depolarized bands is found which can be ascribed to VasO-Ge-O modes.

B. A broad polarized band, situated at about 540 cm -1, is found which is ascribed to vsO-Ge-O and deformation modes, comparable to the deforma- tion modes causing a strong polarized band at 412 cm -1 in the Raman spec- trum of vitreous GeO2 (table 2).

C. Below 400 cm -1 , besides a partly depolarized continuum a polarized peak at about 330 cm -~ is found which can be ascribed to enneagermanate structures. It is comparable to the strong peak at 309 cm -1 in the Raman spectrum of crystalline 2 Na20" 9 GeO2. This conclusion anticipates the results for the potassium germanate glasses.

3.2.3. Potassium germanate glasses. Polarized and depolarized Raman spectra of po- tassium germanate glasses are given in figs. 5a and 5b respectively. The Raman spec- trum of 0.333 K20 0.667 GeO2 resembles closely the spectrum of the crystalline compound of the same composition K20 GeO:. The assignment of the spectra has already been given in [61]. The polarized band at 872 cm -1 is ascribed to vGe-O- of digermanate and the weak polarized band at 790 cm -1 to vsGe-O- of some resi- dual metagermanate chains. The broad polarized band at 520 cm -1 is ascribed to VsO-Ge-O modes and the bands observed at 860 and 780 cm -a in the depolariza- tion spectrum of 0.333 K:O 0.667 GeO2 glass to VasO-Ge-O modes.

A polarized uGe-O- band, indicating the presence of NBO's, is found in the spectra of 0.250 KzO 0.750 GeO2 and 0.200 K20 0.800 GeO2 glass at 876 and 874 cm -a respectively. Furthermore the following general spectral features are observed for the potassium germanate glasses:

A. In the 700-1000 cm -1 region a contour of partly depolarized bands is found which is ascribed to VasO-Ge-O modes.

B. In the spectra of 0.250 K:O 0.750 GeO2 and 0.200 K20 0.800 GeO2 glass, shoulders at about 603 and 645 cm -1 are found which are ascribed to VsO-Ge-O or vsGeO6 of enneagermanate structures (2 Na:O - 9 GeO2). As in the case of sodium germanate glass the intensity and peak position of the enneagermanate bands agree remarkably well with the peak position and intensity of the peaks in the same region in the Raman spectrum of 2 Na20 - 9 GeO2. The strongest enneagermanate peak is also visible in the Raman spec- tra of 0.182 K20 0.818 GeO2, 0.150 K20 0.850 GeO2 and 0.111 K20 0.889 GeO2 glass as a shoulder at about 600 cm -1.

C. A broad polarized band at about 540 cm -1 is found which is acribed to Vs)-Ge-O and deformation modes, comparable to the deformation modes in vitreous GeO2 (table 2).

96 H. Verweii, J.H.J.M. Buster /Raman spectra of alkali germanate glasses

(b) Y(ZY)X /

x:0333 \~ J

I i I I I I 1200 1000 800 600 400 200 0 1200- 1000 800 600 400 200

av(cm-1) av(cm-1)

Fig. 5. Polarized (a) and depolarized (b) Raman spectra of xK20 (1 - x) GeO 2 glasses. Nota- tion for the scattering geometry according to [67]. Relative scale expansion of the depolarized Raman spectra: 10X.

D. Below 400 cm -t a partly depolarized continuum is found and a sharp polar- ized peak at about 324 cm-t , the intensity of which seems to be coupled with the enneagermanate peaks at 603 and 645 cm -1.

4. Discussion and conclusions

The hypothesis of the occurrence of 6-coordinated Ge [6,7] has been confirmed in the present Raman study of alkali germanate glasses. In the Raman spectra of lithium, sodium and potassium germanate glasses we found peaks at respectively 560/540,653/600 and 645/603 cm -1 , which can be ascribed to structures as occur in 2 Li20 9 GeO2 [40] and 2 Na20 9 GeO2 [50]. These "enneagermanate" struc- tures consist of chains of GeO4 tetrahedra, connected by GeO6 octahedra. Non- bridging oxygen atoms in xA20 (1 -x ) GeO2 glasses only occur at x>0.18 (enneagermanate composition).

Using the results of the present study and of [61,62] the following scheme for

H. Verweij, J.H.J.M. Buster/Raman spectra of alkali germanate glasses 97

the molecular structure of alkali germanate glasses may be arrived at. 0

98 H. Verweij, J.H.J.M. Buster / Raman spectra of alkali gerrnanate glasses

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